Because the minimum now is 3G (iPhone 5 and above).

Now the only option is Enable LTE (4G). If you feel that the signal is weak, just turn off cellular data to conserve battery.

What I am trying to tell you is, 3G is the minimum you can MANUALLY set your current data speed.

Of course it is about phone models. iPhone 4/4s does not have a LTE chip embedded therefore you can set from 3G to 2G/EDGE. iPhone 5 onwards has LTE chip embedded therefore you can only set between LTE on or off and MANUALLY set to 3G as the MINIMUM.

Your connection drops from 3G to 2G is because your cellular carrier does not have 3G connectivity coverage in that particular area or zone / weak 3G reception that is why your connection dropped from 3G to 2G. Of course if the connection frequently switch between 3G and 2G will drain your phone battery significantly, which is why I suggested that you turn off your cellular data to conserve battery.

I understand perfectly of the question that you are asking, it is you who do not understand what I am trying to explain to you. If you bother to read and digest what I am explaining, you would probably get it by now.

Now 8.3 can switch between lte, 3g and 2g😊👍

Hence i mentioned supposed 

Also subject to carrier, IIRC.